In the field of motor vehicles, it is common to use safety belts comprising a retractor device which is fixed to the floor of the vehicle or in the so-called B-post (i.e. the post which is formed by the bodywork to the side of each front seat), and a seatbelt strap running from the retractor device and across the body of the user. Normally, a guiding device for the strap is arranged at the same level as the shoulder portion of the user. The safety belt is locked in an anchor on the vehicle's floor. In safety belts of the so-called three-point type, the strap is also guided about the anchor, allowing it to run across the waist of the user.
As regards operation of a safety belt, certain demands and requirements exist. Firstly, it is important that the guiding device is arranged so that the strap is guided in a position which is close to the shoulder portion of the user, i.e. in a position which is adapted to the height of the user. In this manner, optimum protection can be provided in the event of a collision. Furthermore, there is a need for a guiding device which provides height adjustment, i.e. which allows individual setting of the position of the guiding point. In particular, there is a demand for an automatic height adjustment device, relieving the user from having to adjust any control or the like in order to set the guiding device in a correct manner.
Furthermore, there is often a requirement that the safety belt should comprise some type of pretensioning device which, in a known manner, is adapted to tension the belt in the event of a collision. In this regard, the strap is intended to be tightened against the upper torso of the user immediately after the collision, i.e. before the user is urged forwards due to his weight. In this manner, the pretensioning device can prevent or reduce personal injuries in connection with collisions. A device for pretensioning a strap by means of a pyrotechnic pretensioner is known from European Patent No. 138,507.
Furthermore, there is a demand for a force-limiting operation of the safety belt in the event of a collision. This means that the safety belt should not be completely unyielding. Instead, it should absorb part of the energy which is developed when the user is urged forwards during heavy retardation.
In recent years, safety belts have been developed in which both the retractor and the guiding device are integrated into the backrest of the vehicle seat (so-called "belt-in-seat" devices), i.e. the retractor of the safety belt is not fixed to the body of the vehicle but instead to the vehicle seat. Such systems provide improved comfort and protection since the strap will always be in the same position in relation to the user, independently of the position of the vehicle seat. Furthermore, a simplified design for the B-post can be provided, as regards the technological demands, the space available and the design, since in this case the B-post does not have to be provided with any belt retractor and guiding device. Finally, a shorter strap can be used, which lowers the costs and decreases the risk for a "film-reel" effect to occur in the retractor. This means that a small amount of the strap is fed out due to the fact that it is packed on the retractor, in spite of the fact that the strap is locked.
Several types of height adjustment devices, pretensioning devices and force-limiting devices are known. However, a problem in connection with today's safety belts resides in the fact that they do not allow a plurality of functions to be integrated into the same construction. This problem implies that today's systems often become space consuming as well as costly. In particular, this is a problem in connection with the socalled "belt-in-seat" devices.